Trials have been made for recording an information signal by changing the orientation of a liquid crystal by use in combination of a liquid crystal material and a photochromic material. For example, there can be given a method of changing a cholesteric liquid crystal phase by mixing a chiral photochromic compound and a liquid crystal material and subjecting the mixture to photoisomerization (C. Denekamp and B. L. Feringa, Adv. Mater., 10, 1082, (1998)), and a method which uses a polymer liquid crystal material containing in the molecule thereof a photochromic compound, of which molecular orientation changes as cis-trans transition occurs under the irradiation with polarized light (M. Eichi and J. H. Wendorff, Macromol. Chem. Rapid Commun., 8, 59, (1987)). These methods, however, have problems with respect to thermal stability in recording, and durability in repeated use, and therefore have not yet been used in practice.
On the other hand, a photochromic diarylethene compound is such a photochromic material that is isomerized in a photon mode and has excellent repeated use durability, so that an optical recording medium using only this compound has been proposed (T. Kawai, et. al., Jpn. J. Appl. Phys., 38 1194, (1999)). However, this method requires exposure to a light with a wavelength range where the diarylethene compound absorbs the light at the time of recording and reading, and the light for use in reading is also used for erasing the record, so that this method has a problem that nondestructive read-out cannot be carried out.
The present invention has been made in order to solve the above-mentioned problem. An object of the present invention is to provide a novel optical recording material having excellent nondestructive read-out performance, repeated use durability and recording preservability, and also to provide an optical recording medium having a recording layer which uses this optical recording material. A further object of the present invention is to provide an optical recording method using this optical recording material, and to provide a method of non-destructively reading out a written record.